AM79C873 Advanced Micro Devices, AM79C873 Datasheet - Page 15

• Manufacturer Part Number: AM79C873
• Description: NetPHY -1 10/100 Mbps Ethernet Physical Layer Single-Chip Transceiver with 100BASE-FX Support
• Manufacturer: Advanced Micro Devices

Scrambler
The scrambler is required to control the radiated emis-
sions (EMI) by spreading the transmit energy across
the frequency spectrum at the media connector and on
Symbol
C
D
R
H
0
1
2
3
4
5
6
7
8
9
A
B
E
F
K
T
V
V
V
V
V
V
V
V
V
V
J
I
Table 1. 4B5B Code Group
Meaning
SFD (1)
SFD (2)
ESD (1)
ESD (2)
Data A
Data B
Data C
Data D
Data E
Data 0
Data 1
Data 2
Data 3
Data 4
Data 5
Data 6
Data 7
Data 8
Data 9
Data F
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Error
Idle
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
4B code
3210
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
0101
0101
P R E L I M I N A R Y
5B Code
43210
11110
01001
10100
10101
01010
01011
01110
01111
10010
10011
10110
10111
11010
11011
11100
11101
11111
11000
10001
01101
00111
00100
00000
00001
00010
00011
00101
00110
01000
01100
10000
11001
Am79C873
the twisted pair cable in 100BASE-TX operation. By
scrambling the data, the total energy presented to the
cable is randomly distributed over a wide frequency
range. Without the scrambler, energy levels on the
cable could peak beyond FCC limitations at frequen-
cies related to repeated 5B sequences like continuous
transmission of IDLE symbols. The scrambler output is
combined with the NRZ 5B data from the code-group
encoder via an XOR logic function. The result is a
scrambled data stream with sufficient randomization to
decrease radiated emissions at critical frequencies.
Since EMI is not a concern in a fiber application, the
scrambler is bypassed in 100BASE-FX.
Parallel-to-Serial Converter
The Parallel-to-Serial Converter receives parallel 5B
scrambled data from the scrambler and serializes it
(i.e., converts it from a parallel to a serial data stream).
The serialized data stream is then presented to the
NRZ-to-NRZI Encoder block
NRZ-to-NRZI Converter
After the transmit data stream has been scrambled and
serialized, the data must be NRZI encoded for compati-
bility with the TP-PMD standard for 100BASE-TX trans-
mission over Category-5 unshielded twisted pair cable.
PECL Driver For 100BASE-FX
The PECL driver accepts NRZI coded data and converts
it to PECL signal levels for transmission over fiber media.
The output pair is a differential pseudo ECL (PECL) in-
terface designed to connect directly to a standard fiber
optic PMD. The differential driver for the FXTD± is cur-
rent mode and is designed to drive resistive termination
in a complementary mode. The FXTD± pins are inca-
pable of sourcing current, this implies that VOH must
be set by the ratios of the Thevenin termination resis-
tors for each of the lines. RIOH is a pull-up resistor con-
nected from the FXTD± output to VCC. RIOL is a pull-
down resistor connected from the FXTD± output to
ground. RIOH and RIOL are electrically in parallel from
an AC standpoint. A target impedance of 50
needed for the transmission line impedance. A value of
62
a Thevenin equivalent characteristic impedance of
49.7
with PECL circuits. VOL is required to be VDD-1.81 or
greater. A sink current of 19 milli-amps (mA) would
achieve this through the output termination resistors.
MLT-3 Converter
The MLT-3 conversion is accomplished by converting the
data stream output from the NRZI encoder into two bi-
nary data streams with alternately phased logic
one events.
for RIOH and a value of 300
and a VOH value of VCC-.88 volts, compatible
for RIOL will yield
15
is